Chemical potential of model benzene fluids using expanded ensemble Monte Carlo simulations

Polarizable empirical force field for aromatic compounds based on the classical drude oscillator

The polarizable empirical CHARMM force field based on the classical Drude oscillator has been extended to the aromatic compounds benzene and toluene. Parameters were optimized for benzene and then transferred directly to toluene, with parameters for the methyl moiety of toluene taken from the previously published work on the alkanes. Optimization of all parameters was performed against an extensive set of quantum mechanical and experimental data. Ab initio data was used for determination of the electrostatic parameters, for the vibrational analysis, and in the optimization of the relative magnitudes of the Lennard-Jones parameters. The absolute values of the Lennard-Jones parameters were determined by comparing computed and experimental heats of vaporization, molecular volumes, free energies of hydration, and dielectric constants. The newly developed parameter set was extensively tested against additional experimental data such as diffusion constants, heat capacities at constant pressure, and isothermal compressibilities including data as a function of temperature. Moreover, the structures of liquid benzene, liquid toluene, and solutions of each in water were studied. In the case of benzene, the computed and experimental total distribution function were compared, with the developed model shown to be in excellent agreement with experiment.

A generalized direct-particle-deletion scheme for the calculation of chemical potential and solubilities of small- and medium-sized molecules in amorphous polymers

The development, validation, and first applications of a generalized version of an inverse Widom method are described. It permits the calculation of solubility coefficients for molecules as large as, e.g., benzene in all polymers for which reasonable forcefield parameters exist. Predicting the solubility is a key to the knowledge-based design of materials utilized to solve permeability related problems. For long time, particle insertion methods, such as the Widom method, were the only way to predict solubilities from molecular models, but they, in most cases, only worked well for rather small penetrants (e.g., H2, O2, N2). Therefore, a few years ago, a new particle deletion algorithm "DPD" was introduced by Boulougouris, Economou, and Theodorou to overcome this problem in principle. The related computer code was, however, only applicable to special, relatively simple model systems. As application examples for the generalized version described here, solubility calculations for nitrogen, oxygen, and benzene in poly(dimethyl siloxane) are presented.

Toward efficient chemical potential calculations by expanded ensemble simulations; to make the free energy pathway fairly level

A scheme is suggested of how to construct good bias potentials ("balancing factors") to be used in expanded ensemble (EE) calculations of chemical potentials of solutions. A combination of two strategies are used: (i) to use a pathway for particle insertions that avoids large variations in free energy and (ii) to use calculated free energy derivatives to construct a bias potential that makes the pathway fairly level. Only a few very short simulations are needed to accomplish the latter, and then, a full EE simulation is done to obtain the chemical potential. By practical calculations of the chemical potential of benzene, cyclohexane, and benzylamine in water, it is shown that this method is at least equally efficient to the recent adaptive EE (AEE) method by Aberg et al. (J. Chem. Phys. 2004, 120, 3370). Furthermore, the new method provides an alternative strategy that complements existing EE methods.

Vapor−Liquid and Vapor−Solid Phase Equilibria for United-Atom Benzene Models near Their Triple Points: The Importance of Quadrupolar Interactions

Gibbs ensemble Monte Carlo simulations were used to calculate the vapor-liquid and vapor-solid coexistence curves for benzene using two simple united-atom models. An extension of the Gibbs ensemble method that makes use of an elongated box containing a slab of the condensed phase with a vapor phase along one axis was employed for the simulations of the vapor-solid equilibria and the vapor-liquid equilibria at very low reduced temperatures. Configurational-bias and aggregation-volume-bias Monte Carlo techniques were applied to improve the sampling of particle transfers between the two simulation boxes and between the vapor and condensed-phase regions of the elongated box. An isotropic united-atom representation with six Lennard-Jones sites at the positions of the carbon atoms was used for both force fields, but one model contained three additional out-of-plane partial charge sites to explicitly represent benzene's quadrupolar interactions. Both models were fitted to reproduce the critical temperature and density of benzene and yield a fair representation of the vapor-liquid coexistence curve. In contrast, differences between the models are very large for the vapor-solid coexistence curve. In particular, the lack of explicit quadrupolar interactions for the 6-site model greatly reduces the energetic differences between liquid and solid phases, and this model yields a triple point temperature that is about a factor of 2 too low. In contrast, the 9-site model predicts a triple point of benzene at T = 253 +/- 6 K and p = 2.3 +/- 0.8 kPa in satisfactory agreement with the experimental data (T = 278.7 K and p = 4.785 kPa).

Determination of solvation free energies by adaptive expanded ensemble molecular dynamics

A new method of calculating absolute free energies is presented. It was developed as an extension to the expanded ensemble molecular dynamics scheme and uses probability density estimation to continuously optimize the expanded ensemble parameters. The new method is much faster as it removes the time-consuming and expertise-requiring step of determining balancing factors. Its efficiency and accuracy are demonstrated for the dissolution of three qualitatively very different chemical species in water: methane, ionic salts, and benzylamine. A recently suggested optimization scheme by Wang and Landau [Phys. Rev. Lett. 86, 2050 (2001)] was also implemented and found to be computationally less efficient than the proposed adaptive expanded ensemble method.